Control of liquid dissemination

ABSTRACT

A polymer prepared by the addition polymerisation of an alkyl styrene is dissolved in a liquid hydrocarbon fuel which has a flash point of at least 90*F and is of a type suitable for gas turbine aircraft engines in order to reduce the tendency of the fuel to particulate dissemination when the fuel is subjected to shock. Preferred polymers are the non-polar homopolymers or copolymers of alkyl styrenes in which the alkyl group contains from 3 to 20, and more preferably from 3 to 8, carbon atoms. The polymer used has a viscosity average molecular weight greater than 106 or an intrinsic viscosity greater than 2.5 dls./gm. and the amount of polymer is such that there is molecular overlap of polymer molecules in the liquid.

United States Patent 1191 Osmond et a1.

1 1 CONTROL OF LIQUID DISSEMINATION [73] Assignee: Imperial Chemical Industries Limited, London, England 22 Filed: Aug. 2, 1971 211 Appl. N; 168,412

Related US. Application Data [63] Continuation-impart of Ser. No. 818,249, April 22, 1969, which is a continuation-in-part of Ser. No. 786,629, Dec. 24, 1968, abandoned, which is a continuation-in-part of Ser. No. 751,992, Aug. 12,

1968, abandoned.

[] Foreign Application Priority Data May 13, 1971 United Kingdom 14641/71 Apr. 11, 1968 United Kingdom 17544/68 [52] US. Cl 44/62; 44/80 [51] Int. Cl C101 l/l6 [58] Field of Search 44/62, 80, 7 D, 7 E

[56] References Cited UNITED STATES PATENTS 3,136,743 6/1964 Conway et a1. 252/56 R 3,231,498 1/1966 de Vries 44/62 3,326,804 1/1957 Shih-en Hu 44/62 3,473,901 8/1969 de Bonneville et a1. 44/62 3,527,582 9/1970 Haigh et a1. 44/62 OTHER PUBLICATIONS Defensive Publication T858,018, Jacobson, N., Multifunctional Polymeric Additive for Mineral Oils, Jan. 21, 1969, Application Ser. No. 664,925.

[ Dec. 9, 1975 Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Vol. 15, 1968, page 87.

Tanford, Physical Chemistry of Macromolecules, Wiley 1961, pp. l68; 174-178; l98200; 402406.

Miller, The Structure of Polymers, Reinhold 1966, pp. -193;2l3,2l4.

Porter et al., The Entanglement Concept in Polymer Systems," Chemical Reviews, Vol. 66, No. l, 1966, pp. 1 4,13.

Cohen et al., Journal of Polymer Science, Vol. 49, 1961, pp. 377383.

Flory, Principles of Polymer Chemistry, Cornell U., 1953, pp. 610-611.

Primary ExaminerDaniel E. Wyman Assistant ExaminerY. H. Smith Attorney, Agent, or FirmCushman, Darby & Cushman [57] ABSTRACT A polymer prepared by the addition polymerisation of an alkyl styrene is dissolved in a liquid hydrocarbon fuel which has a flash point of at least 90F and is of a type suitable for gas turbine aircraft engines in order to reduce the tendency of the fuel to particulate dissemination when the fuel is subjected to shock. Preferred polymers are the non-polar homopolymers or copolymers of alkyl styrenes in which the alkyl group contains from 3 to 20, and more preferably from 3 to 8, carbon atoms.

The polymer used has a viscosity average molecular weight greater than 10 or an intrinsic viscosity greater than 2.5 dls./gm. and the amount of polymer is such that there is molecular overlap of polymer molecules in the liquid.

10 Claims, No Drawings CONTROL OF LIQUID DISSEMINATION This application is a continuation-in-part of Ser. No. 818,249 filed Apr. 22, 1969, which is a continuationin-part of Ser. No. 786,629 filed Dec. 24, i968, which is a continuation-in-part of Ser. No. 751,992 filed Aug. 12, 1968, both now abandoned.

This invention relates to improved liquid hydrocarhon aircraft fuels.

lt is known that when a liquid with a free surface is subjected to conditions of shock there is a tendency for the liquid to become disseminated in particulate form and that the effect of shock may be such as to convert a proportion of the liquid into a dispersion of fine liquid droplets in air, i.e. a mist.

It is very desirable with aircraft fuels to be able to control the extent to which such a dispersion or mist of the fuel is formed under shock conditions since, in the presence of ignition sources, the mist is likely to ignite and constitute a hazard. In particular it is most important to keep to a minimum the formation of such mist under the shock conditions which occur when the aircraft crashes. Though hydrocarbon fuels now used for aircraft gas turbine engines may be of a higher flash point than aviation gasoline as used in spark-ignition engines with a consequent reduction in the risk of fire due to ignition of vapour, mists of fuels with flash points of 90F. and higher are still highly susceptible to ignition by flames, electrical sparking or the effect of friction, as well as by the presence of hot metal in the engines, and so there is still a considerable fire hazard immediately after a crash of an aircraft using such fuel. Furthermore, there is the risk of propagation of fire to the bulk of liquid fuel even if little damage is caused by ignition of the mist itself.

It is an object of this invention therefore to reduce the tendency to particulate dissemination on being subjected to shock, of a liquid hydrocarbon fuel suitable for use in gas turbine engined aircraft and having a flash point of at least 90F.

We have now found that when such a liquid hydrocarbon fuel having a free surface is subjected to conditions of shock as herein described, the particulate dissemination of the liquid is reduced when there is dissolved in the liquid a polymer of molecular weight greater than (viscosity average) or of intrinsic viscosity greater than 2.5 dls./gm. in a concentration such that there is molecular overlap of the dissolved polymer.

The term molecular overlap" describes the condition in which the segment density of the dissolved polymer in the liquid is substantially uniform on a molecular scale. This condition corresponds to concentrations at and above that at which the centres of mass of the polymer molecules are spaced, on average, at twice the radius of gyration of the polymer molecules. At lower concentrations the polymer molecules may be partially overlapped but the segment density on a molecular scale varies between a maximum value at the centre of mass of a molecule and a minimum value midway between the centres of mass of adjacent molecules.

The segment density distributions in individual polymer molecules are known or can be calculated by formula as given in Physical Chemistry of Macromolecules by Tanford (John Wiley, 1963) p.176. Consequently the lowest concentration at which molecular overlap occurs can be calculated. Where intrinsic viscosity rather than molecular weight is known an alternative calculation can be made using the formula given in Polymer Chemistry" by Flory (Cornell U.P.l953) p.61 1. Further, the lowest concentration at which this condition occurs may also be determined by plotting on log/log scales the apparent viscosity at zero rate of shear of polymer solutions against polymer concentration by weight. We find that such plots in the region of the concentration at which the desired molecular overlap first occurs, i.e. in the region 0.0l to l percent of polymer, consist essentially of two straight lines intersecting at the critical concentration.

By the expression subjected to conditions of shock we mean subjected to external forces operating for only a short time, i.e. to impulses as understood in classical applied mathematics, such that the shape of the liquid undergoes rapid deformation.

Typical conditions of shock may arise for example from:

a. the impact ofa falling or projected mass of the liquid with a rigid surface;

b. the impingement of a solid or liquid mass on a free surface of the liquid;

c. the application of a force to a wall of an open or vented container for the liquid such as to produce transient deformation of the container;

d. the direct exposure of a free surface of the liquid to a rapid fluid flow, for example when a stream of the liquid is ejected into a turbulent air stream.

In these conditions of shock the liquid is subjected to a high rate of increase in rate of shear.

Liquids having a surface tension less than or not substantially greater than that of water and which have a viscosity less than l0 poises, and especially less than 1 poise, when subjected to shock conditions as defined, may produce substantial quantities of finely divided droplets from a free surface even when the shock is as little as that produced in dropping a sample, say 5 gms., of the liquid from a height of several inches onto a rigid surface. By means of this invention a reduction in dissemination of the said liquid hydrocarbon fuel may be obtained under such mild conditions of shock and under much more severe conditions.

Liquid hydrocarbon fuels suitable for use in gas turbine engined aircraft contain antioxidants such as:

a. N,N' -diisopropyl-para-phenylenediamine b. N,N'-disecondary butyl-paraphenylenediamine c. 2,6-ditertiary butyl-4-methylphenol d. 2,4-dimethyl-6-tertiary butylphenol e. 2,6-ditertiary butylphenol f. percent min. 2,6-ditertiary butylphenol 25 percent max. tertiary and tritertiary butylphenols g. 72 percent min. 2,4-dimethyl-6-tertiary butylphenol 28 percent max. monomethyl and dimethyl tertiary butylphenol h. 65 percent min. N,N-disecondary butyl-paraphenylenediamine 35 percent max. N,N'-disecondary butyl-orthophenylenediamine These materials are usually present in a proportion of not more than 24 mg./liter and preferably at least 8.6 mg./liter.

The fuel may also contain:

metal deactivator such as N,N'-disalicylidene-l,2 propane diamine in amount not exceeding 5.8 mgjliter', corrosion inhibitor. A relevant U.S. Mili- 3 tary Specification for fuel-soluble corrosion inhibitors is MlL-l-25017', icing inhibitor, such as ethylene glycol monomethyl ether or a mixture thereof with glycerol. A suitable proportion is from 0. l to 0.15 percent by volume of the fuel. A relevant US. Military Specification for fuel system icing inhibitors is MlL-l-27686; antistatic additive such as Shell Antistatic Additive ASA-3 in a concentration not exceeding p.p.m. By use of this additive the electrical conductivity of the fuel may be brought within the range 50 300 picomhos/metre. For use in this invention the liquid hydrocarbon fuel should have a flash point of at least 90F. as determined by Test Method ASTM Standard D93.

Suitable liquid hydrocarbon fuels to which this invention may be applied include Aviation Turbine Fuels Grade JP-S (flash point 110F. min.) as specified in U.S. Military Specification MlL-T-83l33, Grade JP-S (flash point l40F. min) as specified in US. Military Specification MlL-T-5624G, Grades Jet A and Jet A-l (flash point 1 10F. min.) as specified in ASTM Specification D.1655/66T and Grade AVTUR-NATO Code No. F-35 (flash point 100F. min.) as specified in UK. Ministry of Aviation Specification No. D.Eng.R.D.2494 (Issue 4).

The nature of the polymer to be dissolved in the liquid hydrocarbon fuel which may be subjected to shock conditions is limited primarily by its molecular weight and we find that in order to influence the other characteristics of the liquid to a minimum extent and to reduce dissemination over the widest range of shock conditions the molecular weight should be greater than 10 (viscosity average). In the case of hydrocarbon polymers, this lower limiting molecular weight of 10 (viscosity average) corresponds to an intrinsic viscosity of 2.5 dls/gm. as determined in a hydrocarbon liquid in which the polymer is soluble at C., and in the case of polymers for which the constants necessary for the calculation of viscosity average molecular weight are not readily available this value of intrinsic viscosity may be accepted as a corresponding lower limit.

ln view of the requirement for high molecular weight the most suitable polymers are those prepared by addition polymerisation using free-radical, ionic, Ziegler and other types of initiators.

The most suitable polymers are alkylated polystyrenes which have been prepared by the addition polymerisation of the corresponding alkyl styrene, for example using a free radical initiator or an ionic initiator. These polymers, in contrast to polymers obtained by a Friedel Crafts condensation reaction, have a substantially uniform distribution of alkyl groups and hence they have improved solubility characteristics. Moreover, the use of the addition polymerisation process permits the introduction of comonomers which can be selected to modify the solvency of the final polymer.

The polymers which we find useful are the non-polar homopolymers and copolymers of alkyl styrenes in which the alkyl group contains from 3 to 20 carbon atoms.

Suitable polymers are prepared by homopolymerisation or copolymerisation of, for example, n-propyl styrene, n-butyl styrene, n-amyl styrene, n-hexyl styrene and n-dodecyl styrene; sec.-butyl styrene, sec.amyl styrene, sec.-hexyl styrene and sec.-dodecyl styrene; iso-propyl styrene, iso butyl styrene, iso-amyl styrene,

iso-hexyl styrene and iso-dodecyl styrene; tert-butyl styrene, tert-amyl styrene, tert-hexyl styrene and tertdodecyl styrene.

These monomers may be copolymerised with a wide range of other non-polar monomers for example styrene and vinyl toluene.

Preferably the alkyl group contains from 3 to 8 carbon atoms.

Particularly preferred polymers are the hompolymers and copolymers of C alkyl-styrenes, for example tert-butyl styrene.

The polymers may be made by conventional methods, for example using free radical on an ionic initiator but one particularly suitable method is by aqueous emulsion polymerisation under conditions which provide the necessary high molecular weight. Preferably this polymerisation is carried out at as low a temperature as possible in the presence of a redox initiator system, for example ammonium persulphate and sodium dithionite. Solution polymerisation may also be used.

Polymer particles obtained by emulsion polymerisation may be isolated and then dissolved in the liquid hydrocarbon. Alternatively the aqueous dispersion of polymer particles may be added to the fuel, the water removed and the polymer simultaneously dissolved by appropriate heating. It is clearly advantageous to prepare the polymer is such particulate form as compared with the preparation of polymer in solution. In the latter case the polymer can only be isolated in a bulk form which may be intractable and dissolve only slowly in the hydrocarbon.

The poly (alkyl styrenes) prepared by addition polymerisation have good solubility in liquid hydrocarbon fuels in general and in particular the fuels specified hereinbefore, namely, Grade JP-8, Grade .lP-5, Grade Jet A, Grade Jet A-1 and Grade Avtur.

At the low concentrations required to minimise misting under crash conditions, for example 0.05 1 percent by weight, dissolution of polymer in the fuel can often be effected, unless water is present, without the application of heat.

In general the solubility of the polymer in the liquid hydrocarbon fuel should be such that the theta-temperature of the system is below the temperature to which the solution is likely to be subjected, otherwise there is a danger of precipitation of the polymer. The polymersolvent relationship at the theta-temperature is discussed by P. J. Flory in Principles of Polymer Chemistry at pages 612-615. In jet aircraft the lowest temperature to which the fuel is likely to be subjected is about --50C.

It is an advantage that when the dissemination of the liquid need no longer be reduced, for example when a liquid fuel is to be sprayed into the combustion chamber of a gas turbine engine, the molecules of the polymer used in the invention may be readily reduced in molecular weight by a suitable degradation process, for example by mechanical shearing, or disentangled by shear thinning. Such treatment, leading to a decrease in control of dissemination of the liquid may be carried out in the engine itself or at an earlier stage in the feed line to the engine.

It is also an advantage that the polymers are normally inert and are used in such minor concentration that they have a minimal effect on the important characteristics of the liquid fuel, for example, the calorific value and non-gumming and non-corrosive characteristics.

The concentration of polymer dissolved in the liquid hydrocarbon fuel is broadly determined by the requirement that there should be molecular overlap of the dissolved polymer. In practice one way in which the desired minimum concentration at which this condition exists may be experimentally determined is by measuring the viscosity of a range of solutions of a polymer in the liquid over a range of shear rates. Suitable apparatus to use for this measure is a Contraves Rheomet or a Weissenburg Rheogoniometer. An apparent viscosity at zero shear rate is then obtained by arbitrarily extrapolating the values at each polymer concentration to zero shear rate, these values then being plotted again the corresponding concentration. Such plots, when on log/log scales, consist essentially of two straight line portions, the intersection of which shows up a critical region of concentration for each molecular weight in which a more rapid increase in viscosity begins to take place.

We have found that when the polymer has a molecular weight of above (viscosity average) or an intrinsic viscosity of greater than 2.5 dls./gm. a marked reduction in shock dissemination of the solution is obtainable at polymer concentrations as low as those in these critical regions where there is an upturn in the log/log plots of viscosity (apparent at zero shear rate) against concentration. It is known of course that shock dissemination of liquids can be reduced by drastically increasing their viscosity but the surprising result of application of this invention is that a significant reduction in shock dissemination is obtainable long before the equilibrium low shear viscosity of the solution has been raised to the high value one would expect to need to reduce shock dissemination of the solution. The resistance of the liquid to shock dissemination increases as the proportion of polymer dissolved therein increases but so does the equilibrium low shear rate viscosity of the liquid until it reaches a point where significant resistance to shock dissemination would be expected simply because this viscosity value is sufficiently high. Using the selected polymers of this invention a marked reduction in shock dissemination of liquids can be obtained when the solution of the polymer in the liquid has an equilibrium low shear rate viscosity of less than 100 centipoises.

The proportion of polymer required in any particular case will depend on its molecular weight; the higher the molecular weight of the polymer, the lower will be the proportion of the polymer required to achieve a specified anti-misting effect. As stated above, the reduction in shock dissemination begins at concentrations corresponding to about the upturn in the log/log plot of viscosity (apparent viscosity at zero shear rate) against concentration and increases thereafter.

We have found that a useful empirical test which gives an approximate indication of the concentration at which this upturn in viscosity, resulting from molecular overlap of the polymer molecules in the solution which results in the reduction in shock dissemination of the liquid, is one in which a thin stream of the solution is dropped into the centre of a hollow cylindrical metal vessel the wall of which is lined with absorbent paper. Any splashes of liquid falling on the paper can readily be detected if a small quantity of soluble dye is added to the solution. If a 10 ml. sample of the solution is dropped in a thin stream from a height of 2 metres into such a vessel 17 cm. in diameter and 21 cm. high then the minimum polymer concentration at which no liquid 6 splashes onto the paper lining the wall corresponds approximately to the upturn concentration on the log/log viscosity/concentration curve of the solution, i.e. the minimum concentration for molecular overlap.

This is the minimum useful anti-misting concentration of any polymer and we have found that in practice the most useful concentrations lie in the range 1.5-15 times this minimum and preferably are about 2 10 times this minimum.

As a general indication of practically useful concentration ranges, a polymer of molecular weight (viscosity average) about 10 (corresponding to an intrinsic viscosity of about 10 dls./gm.) has a useful effect in aircraft fuels at a concentration of as low as 0.05 percent by weight whereas in the case of a polymer of viscosity average molecular weight about 10 (corresponding to an intrinsic viscosity of about 2.5 dls./ gm.) a concentration of about 1 percent by weight is desirable. Preferably the aircraft fuels of this invention contain from 0.1 to 2 percent by weight of an appropriate polymer of molecular weight (viscosity average) at least 10 or of intrinsic viscosity at least 2.5 dls./gm.

High molecular weight polymers usually are a mix- 'ture of polymers of a range of molecular weight or intrinsic viscosities, a range which is sometimes very wide. However, this invention makes use of the efiect of polymers of viscosity molecular weight greater than 10 or of intrinsic viscosity greater than 2.5 dls./gm. and although these selected polymers are effective and can be used in the presence of polymer of lower molecular weight or intrinsic viscosity the lower polymers cause an increase in viscosity of the liquid without the same beneficial effect on resistance to shock dissemination as the higher polymers.

When dissolving polymer in the liquid care should be taken to avoid degradation of the polymer.

The soluble polymers of this invention may, if desired, be employed in conjunction with particulate dispersions or other methods of liquid modification or gelation may be employed without losing the benefits of the invention.

The invention is illustrated by the following example in which all parts and percentages are by weight.

EXAMPLE l A mixture of 1,152 parts of demineralised water, 288 parts of acetone, 9 parts of the surfactant Manoxol" OT (a commercially available sodium dioctyl sulphosuccinate; Manoxol is a Registered Trade Mark) and 360 parts of tertiary butyl styrene was stirred under nitrogen at 25C until the surfactant had dissolved. A two-component addition polymerisation initiator consisting of (a) a solution ofO. 18 parts of ammonium persulphate in 8 parts of demineralised water and (b) a solution of 0.312 parts of sodium dithionite in 8 parts of demineralised water was freshly prepared and introduced rapidly into the mixture. A slow exothermic reaction was observed after about A hour and allowed to continue for 6 8 hours the reaction temperature reaching a peak of 35C during this time.

The product was a dispersion of poly(tertiary-butyl styrene) which had a solids content of 18 20 percent by weight and the average particle diameter of the dispersion was 0.1 microns. The polymer had a molecular weight (viscosity average) of 20 X 10'.

A 2 percent solution of poly(tertiary-butyl styrene) in a typical aviation fuel, AVTUR 50 (Defence Specification D.Eng. RD 2494) was prepared by stirring the 7 appropriate proportion of the above dispersion and AVTUR 50 at 150C and removing the water by azeotropic distillation. This solution behaved as a viscous, viscoelastic gel.

A series of solutions of the polymer ranging from 0.05 to 1 percent polymer concentration was prepared by dilution of the 2 percent solution with further AVTUR 50. These solutions were subjected to the splash test described hereinbefore in which there is observed the pattern of liquid splashes produced by allowing the modified fuel (coloured by addition of a dyestuff) to fall into a hollow cylindrical vessel lined with absorbent paper. At 0.l percent polymer concen tration there was a spatter pattern of medium size spots and low frequency and at 0.2 percent polymer concentration there was complete absence of spatter.

The solutions were also tested for their resistance to misting and ignition in aircraft crash conditions by the following test method.

The test apparatus consists of a propulsion unit capable of accelerating a small trolley guided along a track to a speed of approximately 120 feet/sec. The trolley is coupled to a braking system which is capable of stopping the trolley at a mean deceleration of 30 times the acceleration of gravity. A fuel tank is attached to the trolley and at the forward end of the fuel tank is an orifice which is closed with a weighted rubber bung. Approximately 45 rnls. of the fuel to be tested are placed in the tank and the trolley is winched back to a release point from which it is released and accelerated up to a speed of 120 feet/sec. The acceleration takes place along about 10 feet of the track and the trolley is then decelerated along about 10 feet of the track by the braking system so that the weighted bung is ejected and the fuel is expelled through the tank orifice.

There is an ignition array of small gas flames spaced linearly at one foot intervals beneath the portion of the track over which deceleration takes place and beyond the track.

When unmodified AVTUR fuel is subjected to the test it produces a flare above the ignition array of 6 7 feet in length and of large volume. On the other hand, when modified AVTUR fuel according to this Example was subjected to the same conditions, 0.2 0.3 percent polymer concentration was effective in preventing any substantial ignition of the fuel.

EXAMPLES 2 to 8 In these Examples, Example 1 was repeated in entirety except that, instead of tertiary butyl styrene, the polymers used were Example 2 n-butyl styrene Example 3 n-hexyl styrene Example 4 sec butyl styrene Example 5 iso-hexyl styrene Example 6 tert. hexyl styrene Example 7 copolymer of tert. dodecyl styrene:

styrene (90:10)

8 Example 8 copolymer of tert.dodecyl styrene:

vinyl toluene (90:10) AVTUR fuel when modified with these polymers was subjected to the same tests as in Example 1 and similar 5 results were obtained as with the fuel when modified with tertiary butyl styrene.

We claim:

1. A free-flowing liquid hydrocarbon gas turbine engine aircraft fuel of flash point at least 90F. having a reduced tendency to particulate dissemination on being subjected to shock, the fuel containing dissolved therein a polymer which is soluble in said fuel and comprises an alkylated polystyrene prepared by the addition polymerization of an alkyl styrene, the polymer having a viscosity average molecular weight greater than l0 or of intrinsic viscosity greater than 2.5 dls/gm. and in a concentration such that there is molecular overlap of the polymer molecules in the liquid and the concentration being between 0.1 and 2 percent by weight.

2. A liquid fuel as claimed in claim 1 whose thetatemperature is lower than 50C.

3. A liquid fuel as claimed in claim 1 in which the proportion of dissolved polymer is from 0.1 to 2 percent by weight of the liquid.

4. A liquid fuel as claimed in claim 1 in which the proportion of polymer is from 1.5 to 15 times the proportion at which there is an upturn in the log/log plot of apparent viscosity at zero shear rate against polymer concentration in respect of solutions of said polymer in said liquid fuel, in the concentration range 0.0l to 1 percent by weight of polymer.

5. A liquid fuel as claimed in claim 1 in which the proportion of polymer is from 2 to 10 times the proportion at which there is an upturn in the log/log plot of apparent viscosity at zero shear rate against polymer concentration in respect of solutions of said polymer in said liquid fuel, in the concentration range 0.01 to 1 percent by weight of polymer.

6. A liquid fuel as claimed in claim 1 in which the polymer is a non-polar homopolymer or copolymer of an alkyl styrene in which the alkyl group contains from 3 to 20 carbon atoms.

7. A liquid fuel as claimed in claim 1 in which the polymer is a non-polar homopolymer or copolymer of an alkyl styrene in which the alkyl group contains from 3 to 8 carbon atoms.

8. A liquid fuel as claimed in claim 1 in which the polymer is a homopolymer or copolymer of an alkyl styrene in which the alkyl group contains either 3 to 4 carbon atoms.

9. A liquid fuel as claimed in claim 1 in which the polymer is a homopolymer or copolymer of tertbutyl styrene.

10. A liquid fuel as claimed in claim 1 which has been prepared by adding an aqueous dispersion of the polymer to the hydrocarbon liquid and heating the mixture to remove the water and dissolve the polymer. 

1. A FREE-FLOWING LIQUIE HYDROCARBON GAS TURBINE ENGINE AIRCRAFT FUEL OF FLASH POINT AT LEAST 90*F. HAVING A REDUCED TENDENCY TO PARTICULATE DISSEMINATION ON BEING SUBJECTED TO SHOCK, THE FUEL CONTAINING DISSOLVED THEREIN A POLYMER WHICH IS SOLUBLE IN SAID FUEL AND COMPRISES AN ALKYLATED POLYSTYRENE PREPARED BY THE ADDITION POLYMERIZATION OF AN ALKYL STYRENE, THE POLYMER HAVING A VISCOSITY AVERAGE MOLECULAR WEIGHT GREATER THAN 10**6 OR OF INTRINSIC VISCOSITY GREATER THAN 2.5 DLS/GM. AND IN A CONCENTRATION SUCH THAT THERE IS MOLECULAR OVERLAP OF THE POLYMER MOLECULES IN THE LIQUIE AND THE CONCENTRATION BEING BETWEEN 0.1 AND 2 PERCENT BY WEIGHT.
 2. A liquid fuel as claimed in claim 1 whose theta-temperature is lower than -50*C.
 3. A liquid fuel as claimed in claim 1 in which the proportion of dissolved polymer is from 0.1 to 2 percent by weight of the liquid.
 4. A liquid fuel as claimed in claim 1 in which the proPortion of polymer is from 1.5 to 15 times the proportion at which there is an upturn in the log/log plot of apparent viscosity at zero shear rate against polymer concentration in respect of solutions of said polymer in said liquid fuel, in the concentration range 0.01 to 1 percent by weight of polymer.
 5. A liquid fuel as claimed in claim 1 in which the proportion of polymer is from 2 to 10 times the proportion at which there is an upturn in the log/log plot of apparent viscosity at zero shear rate against polymer concentration in respect of solutions of said polymer in said liquid fuel, in the concentration range 0.01 to 1 percent by weight of polymer.
 6. A liquid fuel as claimed in claim 1 in which the polymer is a non-polar homopolymer or copolymer of an alkyl styrene in which the alkyl group contains from 3 to 20 carbon atoms.
 7. A liquid fuel as claimed in claim 1 in which the polymer is a non-polar homopolymer or copolymer of an alkyl styrene in which the alkyl group contains from 3 to 8 carbon atoms.
 8. A liquid fuel as claimed in claim 1 in which the polymer is a homopolymer or copolymer of an alkyl styrene in which the alkyl group contains either 3 to 4 carbon atoms.
 9. A liquid fuel as claimed in claim 1 in which the polymer is a homopolymer or copolymer of tertbutyl styrene.
 10. A liquid fuel as claimed in claim 1 which has been prepared by adding an aqueous dispersion of the polymer to the hydrocarbon liquid and heating the mixture to remove the water and dissolve the polymer. 